This invention relates generally to bore-hole and well mapping navigation, and more particularly concerns apparatus and method to remotely determine tilt from vertical, in a bore-hole.
At the present time it is customary to employ three-axis accelerometer packages or assemblies to accurately determine tilt in a bore-hole. In general, three accelerometers are required, and are mounted in mutually orthogonal relationship to measure gravity components in the Z-direction of the hole axis, and also in X and Y directions at right angles to one another and also perpendicular to the Z axis. The output of each accelerometer is then measured, and a resultant vector constructed to determine the direction of tilt. For low tilt angles (i.e. near vertical) the outputs from the X and Y direction sensing accelerometers provide the useful signal, whereas for high tilt angles the output from the Z direction sensing accelerometer becomes the most sensitive and accurate.
U.S. Pat. No. 3,753,296 to Donald H. Van Steenwyk describes a technique whereas a single accelerometer is rotated about the Z axis, that accelerometer having its tilt sensitive axis perpendicular to the Z axis and thereby sweeping through the X and Y field directions upon rotation. Such rotation enables one accelerometer to take the place of the two (X and Y direction sensing) accelerometers described above. Besides eliminating fixed errors and bias errors, such a rotary or carouseling arrangement realizes many other advantages inasmuch as, depending upon the speed and accuracy of rotation, statistical leverage can significantly improve tilt determination. Thus, improved accuracy, lower cost and simplified tilt measurement can be realized. However, for very high tilt angles, such accuracy rapidly diminishes, and a significant problem remains.